Returned

Notes

1) Only the first character of the type argument is significant.
'R' or 'r' indicates that obs1 and obs2 are the observed Right
Ascension and Declination; 'H' or 'h' indicates that they are
Hour Angle (West +ve) and Declination; anything else ('A' or
'a' is recommended) indicates that obs1 and obs2 are Azimuth
(North zero, East is 90 deg) and zenith distance. (Zenith
distance is used rather than elevation in order to reflect the
fact that no allowance is made for depression of the horizon.)
2) The accuracy of the result is limited by the corrections for
refraction. Providing the meteorological parameters are
known accurately and there are no gross local effects, the
predicted apparent RA,Dec should be within about 0.1 arcsec
for a zenith distance of less than 70 degrees. Even at a
topocentric zenith distance of 90 degrees, the accuracy in
elevation should be better than 1 arcmin; useful results
are available for a further 3 degrees, beyond which the
slaRefro routine returns a fixed value of the refraction.
The complementary routines slaAop (or slaAopqk) and slaOap
(or slaOapqk) are self-consistent to better than 1 micro-
arcsecond all over the celestial sphere.
3) It is advisable to take great care with units, as even
unlikely values of the input parameters are accepted and
processed in accordance with the models used.
4) "Observed" Az,El means the position that would be seen by a
perfect theodolite located at the observer. This is
related to the observed HA,Dec via the standard rotation, using
the geodetic latitude (corrected for polar motion), while the
observed HA and RA are related simply through the local
apparent ST. "Observed" RA,Dec or HA,Dec thus means the
position that would be seen by a perfect equatorial located
at the observer and with its polar axis aligned to the
Earth's axis of rotation (n.b. not to the refracted pole).
By removing from the observed place the effects of
atmospheric refraction and diurnal aberration, the
geocentric apparent RA,Dec is obtained.
5) Frequently, mean rather than apparent RA,Dec will be required,
in which case further transformations will be necessary. The
slaAMP etc routines will convert the apparent RA,Dec produced
by the present routine into an "FK5" (J2000) mean place, by
allowing for the Sun's gravitational lens effect, annual
aberration, nutation and precession. Should "FK4" (1950)
coordinates be needed, the routines slaFk425 etc will also
need to be applied.
6) To convert to apparent RA,Dec the coordinates read from a
real telescope, corrections would have to be applied for
encoder zero points, gear and encoder errors, tube flexure,
the position of the rotator axis and the pointing axis
relative to it, non-perpendicularity between the mounting
axes, and finally for the tilt of the azimuth or polar axis
of the mounting (with appropriate corrections for mount
flexures). Some telescopes would, of course, exhibit other
properties which would need to be accounted for at the
appropriate point in the sequence.
7) The star-independent apparent-to-observed-place parameters
in aoprms may be computed by means of the slaAoppa routine.
If nothing has changed significantly except the time, the
slaAoppat routine may be used to perform the requisite
partial recomputation of aoprms.
8) The date argument is UTC expressed as an MJD. This is,
strictly speaking, wrong, because of leap seconds. However,
as long as the delta UT and the UTC are consistent there
are no difficulties, except during a leap second. In this
case, the start of the 61st second of the final minute should
begin a new MJD day and the old pre-leap delta UT should
continue to be used. As the 61st second completes, the MJD
should revert to the start of the day as, simultaneously,
the delta UTC changes by one second to its post-leap new value.
9) The delta UT (UT1-UTC) is tabulated in IERS circulars and
elsewhere. It increases by exactly one second at the end of
each UTC leap second, introduced in order to keep delta UT
within +/- 0.9 seconds.
10) IMPORTANT -- TAKE CARE WITH THE LONGITUDE SIGN CONVENTION.
The longitude required by the present routine is east-positive,
in accordance with geographical convention (and right-handed).
In particular, note that the longitudes returned by the
slaObs routine are west-positive, following astronomical
usage, and must be reversed in sign before use in the present
routine.
11) The polar coordinates xp,yp can be obtained from IERS
circulars and equivalent publications. The maximum amplitude
is about 0.3 arcseconds. If xp,yp values are unavailable,
use xp=yp=0.0. See page B60 of the 1988 Astronomical Almanac
for a definition of the two angles.
12) The height above sea level of the observing station, hm,
can be obtained from the Astronomical Almanac (Section J
in the 1988 edition), or via the routine slaObs. If p,
the pressure in millibars, is available, an adequate
estimate of hm can be obtained from the expression
hm = -8149.9415 * log (p/1013.25);
(See Astrophysical Quantities, C.W.Allen, 3rd edition,
section 52.) Similarly, if the pressure p is not known,
it can be estimated from the height of the observing

station, hm as follows

p = 1013.25 * exp (-hm/8149.9415);
Note, however, that the refraction is proportional to the
pressure and that an accurate p value is important for
precise work.